In general, a combustor used for hot water use and heating, such as a boiler or a hot water heater, are classified into an oil boiler, a gas boiler, an electric boiler and a hot water heater depending on the fuel it is supplied with, and are diversely developed to fit different installation purposes.
Among these combustors, in particular, the gas boiler and the hot water heater generally use a Bunsen Burner or a Premixed Burner to combust gas fuel, and among these the combustion method of the premixed burner is carried out by mixing gas and air at a mixing ratio for optimal combustion state and supplying this mixture (air+gas) to a burner port for combustion.
Also, function of a combustor is evaluated by a turn-down ratio (TDR). The TDR refers to ‘a ratio of maximum gas consumption to minimum gas consumption’ in a gas combustion device in which the gas volume is variably regulated. For instance, if the maximum gas consumption is 24,000 kcal/h and the minimum gas consumption is 8,000 kcal/h, the TDR is 3:1. The TDR is controlled according to the ability to maintain a stable flame under minimum gas consumption condition.
In the gas boiler and the hot water heater, convenience of using hot water and heating increases with larger TDR. That is, if the TDR is small (meaning the minimum gas consumption is high) and the burner is operated for a small hot water and heating load, frequent On/Off of the combustor occurs, thereby deviation during temperature control increases and durability of the apparatus decreases. Therefore, various methods have been developed to increase the TDR applied to a combustor in order to improve aforementioned problems.
Valves which controls gas supply to these types of burners having proportional control are largely divided into electrical modulating gas valve, which is controlled by current value, and pneumatic modulating gas valve, which is controlled by differential pressure generated during air supply.
The pneumatic modulating gas valve controls the amount of gas supplied to the burner through differential pressure generated when air needed for combustion is supplied to the burner by a fan. At this time, the air and gas needed for combustion are mixed in the gas-air mixer and supplied to the burner as a mixture (air+gas).
In a gas-air mixer of a gas burner using such pneumatic modulating gas valve, the primary factor controlling the TDR is a relationship between gas consumption (Q) and differential pressure (ΔP). The general relationship between differential pressure and flow rate of a fluid is as follows:Q=k√(ΔP)
That is, differential pressure needs to be quadrupled in order to double flow rate of a fluid.
Therefore, differential pressure ratio must be 9:1 in order to have a TDR of 3:1, and the differential pressure ratio needs to be 100:1 to have a TDR of 10:1. However, it is impossible to infinitely increase the gas feed pressure.
In order to solve the above problem of being unable to infinitely increase the gas feed pressure, the present invention describes, as illustrated in FIG. 1, a method for increasing the turn-down ratio of the gas burner by dividing the gas and air supply paths into two or more sections, respectively, and opening/closing each passage of gas injected into the burner.